1. Field of the Invention
The present invention relates to thermoelectric conversion materials and thermoelectric conversion elements.
2. Description of Related Art
Thermoelectric generation is a technology for directly converting thermal energy into electrical energy using the Seebeck effect, i.e. an effect of generating a thermoelectromotive force in proportion to a temperature difference by providing the temperature difference between opposite ends of a substance. This technology has been put to practical use as, for example, a remote area power supply, an outer space power supply, and a military power supply, in some fields. Furthermore, thermoelectric cooling is a technology using the Peltier effect, i.e. a phenomenon in which electrons carried by a current flow can transfer heat. Specifically, these technologies achieve electrical power generation or absorption of heat from the junction of two substances having carriers of opposite signs by utilizing the fact that when the two substances are connected with each other thermally in parallel and electrically in series and an electric current then is passed therethrough, the difference in sign between the carriers is mirrored to the difference in the direction of the heat flow. Examples of the two substances having carriers of opposite signs that can be used include a p-type semiconductor in which the electroconductive carrier is a hole and an n-type semiconductor in which the electroconductive carrier is an electron. The configuration of the above-mentioned element, which is a configuration for achieving thermoelectric generation and thermoelectric cooling, is referred to as a π-type and is the most general configuration.
Generally, the performance of a thermoelectric conversion material is evaluated by a figure of merit Z, a figure of merit ZT that is obtained by multiplying a figure of merit Z by absolute temperature to be non-dimensionalized, or an index that is referred to as a power factor PF. The figure of merit ZT is expressed as ZT=S2T/ρκ, where S is a Seebeck coefficient, ρ is electrical resistivity, and κ is thermal conductivity, of a substance. The larger the figure of merit ZT, the higher the thermoelectric performance. As can be understood from the formula, the conditions required for an excellent thermoelectric conversion material are low thermal conductivity and low electrical resistivity.
Currently, an effective material that is used for thermoelectric conversion elements is Bi2Te3 with an electroconductive carrier injected thereinto. Since Bi2Te3 exhibits a performance of approximately ZT=1 at ordinary temperature, it can be said that Bi2Te3 has a thermoelectric performance at a practical level. Furthermore, since Bi2Te3 allows p-type and n-type carriers to be injected easily, the above-mentioned π-type thermoelectric conversion element can be configured by injecting different carriers from each other using Bi2Te3 alone, without using two types of materials. Further, with respect to materials with complicated structures, such as compounds with skutterudite structures and clathrate compounds, for thermoelectric conversion elements, research and development is still ongoing to allow them to be used practically. However, these materials have a problem in that when used in a high temperature region, they are oxidized and thereby suffer from a deteriorated thermoelectric performance.
Recently, in order to overcome the above-mentioned problem, the use of oxide materials as thermoelectric conversion materials is attracting attention. Since oxide materials do not have the problem of deterioration in performance that is caused by oxidation even in a high-temperature environment, they are considered to be candidates for the thermoelectric conversion materials that replace, for example, Bi2Te3. Among oxide materials, a p-type oxide material composed of layered cobalt oxide has been reported to have a high Seebeck coefficient, excellent electrical conductivity, and low thermal conductivity (κ=0.5 to 3 W/mK at room temperature) that are obtained due to the distinctive crystal structure thereof and thereby to have a high thermoelectric performance (see, for example, JP 9-321346 A). Therefore, development of thermoelectric conversion materials and technical development of thermoelectric generation/Peltier devices are being made vigorously.
On the other hand, among the n-type oxide materials, perovskite structures, such as electron-carrier-doped KTaO3 (see, for example, Akihiro Sakai et al., “Thermoelectric Properties of Electron-doped KTaO3”, Proceedings of the Fifth Annual Meeting of the Thermoelectrics Society of Japan, p. 17), SrTiO3 (see, for example, T. Okuda, “Large thermoelectric response of metallic perovskites: Sr1-xLaxTiO3 (0≦x≦0.1)”, Physical Review B, Mar. 1, 2001, Volume 63 113104), CaMnO3, and LaNiO3, as well as electron-doped ZnO having a wurtzite can be considered as candidates for the thermoelectric conversion material. Since these materials each have a relatively high thermoelectric performance, the use thereof as thermoelectric conversion materials that are paired with layered cobalt oxides in thermoelectric conversion elements is being studied.
The perovskite structure typified by KTaO3 or SrTiO3 has an isotropic crystal structure and can have, for example, a cubic, tetragonal, or orthorhombic crystal system according to the combination of elements. The crystal structure of KTaO3 is described as an example with reference to a drawing. FIG. 5 is a schematic view showing the crystal structure of KTaO3. In order to make it easy to compare with the crystal structure of a thermoelectric conversion material according to the present invention described later, FIG. 5 shows a region twice as large as that defined by each crystal axis in a unit cell. That is, FIG. 5 shows eight unit cells. As shown in FIG. 5, a KTaO3 crystal 101 is a cubical crystal with a lattice constant of 3.988 angstroms. Furthermore, a (TaO3)1− octahedron 102 centered on tantalum (Ta) is present in the crystal structure of the KTaO3 crystal 101. Besides the octahedron 102, eight potassium elements 103 are present in such a manner as to surround one octahedron 102.
However, the above-mentioned n-type oxide materials have thermal conductivities of around 5 to 50 W/mK commonly, which are relatively high. Therefore, when these materials are used for a thermoelectric conversion element, a large thermal leak occurs between a hot area and a cold area in the element, which results in a reduction in the temperature difference in the thermoelectric conversion element. This causes a problem in that the thermoelectromotive force that is generated in the thermoelectric conversion element is reduced, which results in power generation loss. In order to prevent such power generation loss, an n-type oxide material with a lower thermal conductivity is required.